1. Field of the Invention
The present invention relates to pollution control systems and, more specifically, to devices for removing pollutants from the effluent of exhaust systems.
2. Description of the Prior Art
Electrostatic precipitators (ESPs) may be used for collecting dust produced by the combustion of coal in generating electricity with commercial electric power boilers. As shown in FIG. 1, ESPs 2 known to the art usually comprise corona electrodes 4, such as long wires, and parallel collection electrodes 6, such as sheet metal plates. In a typical commercial ESP, there are about 50,000 corona electrodes, each about 30 feet long or more, and about 200,000 square feet of collection electrode surface area.
A rectified half-wave or full-wave voltage is applied between the corona electrodes and the collection electrodes. As the voltage reaches a critical value, gasses surrounding the corona electrode break down electrically and produce an avalanche of electrons, thereby forming a "corona" between the electrodes. Moving under the influence of the electric field between the corona and collection electrodes, the velocity of the electrons decrease as they get further from the corona electrodes. This allows electrons to be captured by gas molecules, thereby producing ions which attach to gas-borne particles, such as dust. The particles are then attracted to the collection electrodes by the electric field and the subsequently collected particles are periodically removed from the collection electrodes by rapping the plates.
The power input to an ESP is limited because the ions and the charged particles must pass through the dust layer on the collection electrodes. If the electrical resistivity of the dust is high, the interstitial gasses in the collected dust layer break down electrically when the current is increased above a critical value. This disadvantageous breakdown is referred to as "back corona" and results in positive ions being generated and propelled into the interelectrode space, which may discharge the previously charged particles and cause sparks between the electrodes. Thus, with high resistivity dust, the current is limited so that the collection efficiency is seriously reduced.
Formation of the corona at the corona electrode occurs first at the point along the electrode with the smallest effective radius, producing a local flare as the voltage is increased. The intensity and length of the flare increases until the space charge generated by the ion cloud and charged particles suppress the corona, causing breakdown at the next smallest radius. This process continues until there are a series of discrete flares or corona points along the length of the corona electrode.
Several studies of the distribution of current through the collected dust layer have shown that the highest current density occurs at the point on the dust layer immediately across from a flare and decreases with distance away from the flare. The ratio of peak to average current is approximately two to one. It is peak value of current density that determines the onset of back corona or sparking. Therefore, significant improvement in ESP performance will occur if a more uniform corona is produced, with a peak current density less than a predetermined maximum.
An alternative to rectified sine wave voltage electrification is the application of a pulsed voltage. A number of commercial installations use voltage pulses with a fast voltage rise time and a short pulse duration (typically one microsecond). This results in a much more uniform corona that typically appears as a uniform sheath surrounding the corona wire. With pulsed energization, currents of about twice that of conventional energization can be attained without sparking or the onset of back corona.
The electrical characteristics of a precipitator can be represented by a resistor-capacitor equivalent circuit, with the capacitor parallel to a variable resistor. When a pulsed voltage is applied, the voltage does not fall at the end of the pulse because it is maintained by the charge on the precipitator capacitance. To achieve a pulse, one must dump the charge into a resistor or similar discharge element. Because the amount of energy dumped is large compared to the useful energy, such type of pulsed energization has the disadvantage of not being operationally economical for most applications.